Metal-doped zeolites are known from the state of the art and are widely used as catalyst material for the purification of exhaust gases.
Because of the harmful effects of nitrogen oxides on the environment, it is important to further reduce these emissions. Clearly lower NOx emission limits for stationary and motor vehicle gases than are customary today are planned in the United States for the near future and are also being discussed in the European Union.
In order to be able to observe these limits, in the case of mobile combustion engines (diesel engines) this can no longer be achieved by measures inside the engine, but only by an exhaust-gas post-treatment, for example with suitable catalysts.
The denitrification of combustion gases is also called DeNOx. In automobile engineering, selective catalytic reduction (SCR) is one of the most important DeNOx techniques. Hydrocarbons (HC-SCR) or ammonia (NH3-SCR) or NH3 precursors such as urea (Ad-Blue®) usually serve as reducing agents. Metal-exchanged zeolites (also called metal-doped zeolites) have proved to be very active catalysts that can be used in a broad SCR temperature range. They are mostly non-toxic and produce less N2O and SO3 than the customary catalysts based on V2O5. In particular iron-doped zeolites represent good alternatives to the normally used vanadium catalysts, because of their high activity and resistance to sulphur under hydrothermal conditions. Customary processes for doping zeolites with metals comprise for example methods such as liquid ion exchange, solid-phase ion exchange, vapour-phase ion exchange, mechanical-chemical processes, impregnation processes and the so-called extra-skeletal processes.
U.S. Pat. No. 5,171,553 discloses for example an ion-exchange process in an aqueous solution wherein silicon-rich zeolites with Si/Al ratios of over 5 to approx. 50 are customarily used as support.
Problems result in particular when doping or introducing active components such as e.g. iron, vanadium, cobalt and nickel into the zeolite, as different oxidation numbers of these catalytically active metals occur next to each other and also the desired catalytically active species is not always obtained, or the catalytically active species change into catalytically inactive species because of the parameters of the doping process (oxygen, temperature, moisture, etc.).
The doping of zeolites with iron by solid-state ion exchange is known (EP 0 955 080 B1), wherein a mixture of the desired zeolite, a metal compound and an ammonium compound is sintered under a protective atmosphere, in particular a reductive protective atmosphere, with the result that metal-containing, in particular iron-doped, catalysts with an increased long-term stability are obtained.
However, it has been shown that in practically all the known processes of the state of the art, cluster species of the catalytically active metals, which are catalytically inactive or greatly reduce the catalytic activity, form as a result of the metal exchange inside the zeolite. By “clusters” are meant polynuclear bridged or unbridged metal compounds which contain at least three identical or different metal atoms. Metal-exchanged zeolites in which no metal clusters were able to be detected inside the zeolite skeleton are thus far unknown.